3α-hydroxy-5α-pregnane-11,20-dione (alfaxalone) is a known anaesthetic for use in a variety of animals. Owing to the fact that in use, alfaxalone has a wide safety margin, rapid induction, high potency, absence of nausea and rapid progress to ambulation, it has been regarded as a very useful anaesthetic. However, the substance is quite water insoluble and therefore must be solubilised for effective parenteral use. Solubilisation in saline has been achieved commercially using polyethoxylated castor oil in combination with a small amount of alfaxalone acetate (21-acetoxy-3α-hydroxy-5α-pregnane-11,20-dione), a steroid which is half as potent an anaesthetic agent as alfaxalone. Nevertheless, its practical usefulness in mammals has been severely limited since these alfaxalone-based compositions invoke a histamine response in a number of species when administered parenterally.
Because of alfaxalone's efficacy, the present inventors have sought to meet the problem of providing an alfaxalone-based composition which is both anaesthetically effective and able to be administered parenterally to mammals without invoking a histamine response.
It is also evident that for practical reasons it is desirable that formulations of water soluble cyclodextrin or cyclodextrin derivative complexes of alfaxalone are presented as ready-to-use solutions. That is no reconstitution is required prior to use. As used in this specification, “complex” is to be understood as referring to the water soluble moiety formed by the hydrophilic/hydrophobic interaction between alfaxalone and cyclodextrin or cyclodextrin derivative.
It may be expected that owing to the common use of phosphate buffer, 0.1M mixed pH 7.0 (BP 1998) in parenteral formulations, cyclodextrin or cyclodextrin derivative complexes of alfaxalone should be suitable for use with this buffer. In fact it has been found by the present inventors that in formulating a ready-to-use anaesthetic composition, there is a fundamental problem in that formulations of the water soluble cyclodextrin or cyclodextrin derivative complexes of alfaxalone with pH 7.0 phosphate buffer are unstable. By unstable it is meant that a crystalline material was formed in a representative formulation within about 7 days when it was stored at 40° C. The presence of such crystals precludes the acceptability in use of such a formulation.